1. Field of the Invention
The present invention relates to a dielectric isolation substrate containing a capacitor.
2. Description of the Conventional Technique
Dielectric isolation substrates containing semiconductor islands, isolation films and a support insulator(s) have been proposed, in which a smoothing capacitor(s) is formed in the dielectric isolation substrate to reduce variations in the power supply voltage A conventional example of a dielectric isolation substrate containing a capacitor is shown in FIG. 1. In this figure, the dielectric isolation substrate has a monocrystal silicon island(s) 1, the rear of which is covered with an isolation layer or film 2 formed of a dielectric film for insulating isolation. One single crystal silicon island forms a silicon island 1a used for forming a capacitor. In this Si-island 1a is formed a region 4 containing a conductive impurity at a high concentration, such as As, Sb or the like at an interface with the isolation film 2 and at an interface with a dielectric film 3 on the surface of the Si island. This high concentration region 4 is one of the capacitor electrodes, the electrode being connected to a wiring or interconnecting layer 6 through a contact hole 5 formed through a dielectric film 3 on the surface of the Si-island. The Si-island 1a is covered with a conductive polycrystalline silicon (polysilicon or poly-Si) layer 7 with the isolation film 2 in between them, the poly-Si layer 7 acting as another electrode for the capacitor. This electrode 7 is connected to another wiring (or interconnecting) layer 9 through a contact hole 8 on the dielectric film 3 on the surface of the Si-island, the wiring layer 9 also formed to extend to the surface of the Si-island 1a. The conductive poly-Si layer 7 is covered with another isolation film 10, and then the Si-island 1a, together with another Si-island 1 is supported integrally by a non-conductive poly-Si layer 11 on the rear side of the dielectric isolation substrate.
However, this conventional technique has a problem The Si-island(s) 1a for a capacitor occupies a large area in an upper surface of the substrate because of the necessity to enlarge the Si-island(s) 1a to increase a capacitance of the capacitor for the purpose of suppressing power supply variations sufficiently.
The present invention has been carried out for eliminating these conventional deficiencies.
Accordingly, an object of the present invention is to provide such a dielectric isolation substrate having formed therein a capacitor(s) that is capable of forming a capacitor of large capacitance without occupying a large area in an upper surface region of the substrate.